Heretofore, a system resorting to affine transformation has been usual in an attempt to rotate two-dimensional image data. Rotating an image through affine transformation requires arithmetic operations based on the following equation (1); ##EQU1## where X, Y are coordinates of the original pixel, x, y are coordinates after transformation and .theta. is an rotated angle of image data. When handling image data such as a displayed image, the above transformation requires to implementation of arithmetic operations of the equation (1) pixel-by-pixel. Thus, a large amount of computation is needed, particularly when rotating a large image and, therefore, it was considered very difficult to obtain a rotated image at high speeds.
Accordingly, as disclosed in Japanese Patent LaidOpen No. 55-94145(1980) by way of example, there has been proposed a system wherein an image is skewed to obtain a rotated image in a pseudo manner, i.e., that an image is first subjected to skew transformation by an angle .theta. in a horizontal direction and then another skew transformation by an angle .theta. in a vertical direction. Such a system resorting to skew transformation by an angle .theta. will be described with reference to FIG. 4. Referring to the figure, 400 is an original image, 401 is the image after skew transformation in a horizontal direction, and 402 is an image derived from 401, which was derived from the original image after the skew transformation in a horizontal direction, after another skew transformation in a vertical direction. These skew transformations can be achieved at high speeds by a suitable method such as skew scan. The skew transformation method can be described by the following equation (2): ##EQU2## where .theta.; minute angle
Because the conventional image rotating system by an arbitrary angle was implemented as mentioned above, approximation of the equation (2) will lose its effectiveness and the rotated image will be distorted, as a rotation angle .theta. increases. As a result, the conventional system can be applied only while the rotation angle is within a range of certain minute angles.